1. Field of the Invention
The present invention relates to devices and methods for detecting substances that are present in liquids. In particular, the invention relates to devices and methods for detecting small molecules, such as chemicals or biological products, that are present in liquid samples derived from body tissues or the environment.
2. Description of Related Art
There are various types of devices and methods available in the art for detecting substances in samples. A large segment of the field utilizes membrane-bound molecules that specifically bind to the substance of interest or to a molecule that is bound to the substance of interest. The two main types of devices and methods are generally referred to as lateral flow and flow through. These tests are generally relatively rapid (less than 1 hour to detect a substance) and sensitive (ng/ml range).
In a flow through device, a sample is pulled through a membrane by capillary action and the substance (analyte, antigen, etc.) is retained on the membrane by binding to a specific antibody, receptor, peptide, etc. The binding is detected by binding of a second antibody or other molecule that is coupled to either an enzyme (e.g., horseradish peroxidase), a colloidal particle (e.g., gold sol), or various other labels and particles (e.g., fluorescent labels, paramagnetic beads). The binding occurs very rapidly as the sample is pulled through the membrane and the membrane is then washed (buffer is pulled through the membrane) and the detection reagent is added. The result is a detectable signal, such as a spot of color, a line, a plus sign, etc.
In a lateral flow device, the sample wicks across a thin membrane by capillary action and flows through a line of a reagent, such as an antibody or other binding component (binding peptides, receptors, etc.). In certain versions, the analyte has already been bound by an antibody with a colored particle attached (e.g., gold sol, blue dextran bead, etc). This complex of antigen and antibody-gold is bound by the reagent line and a colored line appears. There is no washing involved and no liquid reagents are used, except that the sample may be diluted in a buffered solution before it is placed onto the conjugate pad which contains the antibody-gold sol as a dried reagent.
In other versions of the lateral flow device, the sample is usually mixed with a buffer containing an antibody-enzyme conjugate. This is placed onto the membrane, and wicking occurs by lateral flow along the thin membrane. A line is not immediately visible because a reagent must be added. Usually this is a colorless chemical that is converted into an insoluble colored precipitate by the enzyme (e.g., horseradish peroxidase, etc.). This version must be washed to leach away the unbound enzyme, so there is an absorbent pad at each end of the membrane and the membrane is typically more porous than that used for gold-sol lateral flow (this facilitates washing).
Although the devices and methods currently available for detecting substances in liquid samples are suitable and effective for detecting most substances of interest, there is a need for new devices and methods having improved speed, sensitivity, and ease of use.